1. Field of the Invention
The present invention relates to a water drop detector on a transparent substrate for detecting the amount of water drops adhering upon a surface of the transparent substrate by a light emitting element and a light receiving element, and further to an initiating method for the water drop detector used in a wiper driving control device and a stabilizing method for an output therefrom.
2. Description of Related Art
Conventionally, as is described in Japanese Patent Laying-Open No. Hei 2-67945 (1990), there is already known a water drop detector (a rain sensor) for a window glass (windshield) of a vehicle, which comprises a footlight means comprising for example a light emitting element and a detection means comprising for example a light receiving element, wherein light is irradiated from the footlight means towards the windshield, and if an output signal from the detection means which receives reflection light is within a predetermined range over a level for indicating no water drops, it is determined that the water drops stick or adhere upon only the outside of the windshield, while if it exceeds the predetermined range over that level of indicating no water drops, it is determined that the water drops are sticking or adhering upon an interior surface of the windshield.
Further, as a control device applying such a rain sensor, there is already known a wiper control device of a type which is sensitive to the presence of water drops, in which a light emitting means and a light receiving means are provided inside of the windshield, opposite to each other with a distance therebetween, in a wiping area swept by a wiper for wiping off the outside surface of the windshield, whereby it is decided whether water drops are present upon the outside surface of the windshield or not on the basis of the output level of the signal output from the light receiving means, before and after the operation of the wiper, so as to actuate it.
However, with the conventional water drop detector, no temperature compensation is made with respect to the output of the light emitting element, such as a LED (a light emitting diode) nor for the output signal of the light receiving element, such as a PD (a photo diode). Therefore, the output level of the light output from the light emitting element fluctuates depending on the ambient temperature thereof, and also the output signal level from the light receiving element fluctuates depending on the ambient temperature thereof.
Accordingly, if the output signal level from the light receiving element is set with a plurality of levels (for instance, 3-stages or 4-stages) and thereafter driving the wiper(s) depending on the respective output signal levels, i.e., with a low speed, with an intermittent (on and off) operation, or with a high speed, there is a drawback in that effective operation cannot be obtained because of the fluctuation in the output signal level of the light receiving element depending on the ambient temperature.
Further, the light amount which the light receiving element receives from outside changes greatly depending on the environment in which the water drop detection device is used, i.e., it reaches as much as one hundred and fifty thousand (150,000) lux under strong sunshine in mid-summer, for instance, while it reaches only to several lux in darkness at night. With such a change of ambient light conditions, the water drop detector can easily malfunction.
Moreover, with the conventional wiper control device that is sensitive to water drops, there is another drawback in that a wiper driving signal is received even in the case where no water drops adhere upon the outside surface of the windshield, and in particular, when operation of the rain sensor is initiated (i.e., when turning it by switching on an electric source), if water drops adhere on the outside surface of the windshield, the wiper can be driven reciprocally only at one time.
Furthermore, with the conventional wiper control device that is sensitive to water drops, the output signal from the light receiving means fluctuates in the following cases, even when no water drops adhere upon the outside surface of the windshield. For example, one case is where the outside surface of the windshield is contaminated or dirty, another case is where the light emission output fluctuates due to any reason (such as a change in the output of the light emission source with the passage of time), and another case is where the amplification factor of various circuits fluctuates due to variations in the ambient temperature. In addition, noise due to external sources can also cause undesirable results.
Accordingly, the prior art has the drawback that the light receiving means might output a signal indicative that water drops adhere on the outside surface of the windshield in the cases mentioned the above, causing the wiper(s) to be driven when it is not necessary.